When developing complex electronic functions, it is often necessary to connect different printed circuit boards electrically to one another.
To this end, the use of flexible printed circuit boards (FPC Boards) can prove to be of great interest. Indeed, such flexible links can be used, for example, to connect rigid circuits together in complex spatial configurations, thereby gaining space.
In practice, such an FPC Board comprises a layer of conductive tracks included between two layers of flexible, plastic material, for example polyimide, as well as, possibly, a second layer of conductive tracks disposed on one of the external faces of the FPC board (a face intended for example to implement a security mesh as indicated here below). It follows from this that a soldering technique particularly suited to this type of flexible circuit is the thermode or hot-bar soldering technique.
In practice, two types of implementation of this soldering technique can be envisaged. In the first type (called a single-sided flex implementation), only one of the two layers of flexible, plastic material is perforated so as to enable access to the tracks that are to be soldered. In practice, during the soldering, the tracks made accessible are applied to the points of solder paste preliminarily disposed on the tracks of the printed circuit board to which it is sought to solder them. The thermode or hot bar is then applied to the layer of flexible plastic material disposed on the face of the FPC board opposite the perforated face. Thus, solder paste points are soldered by diffusion of heat, on the one hand through the unperforated layer of flexible plastic to which the hot bar is applied and on the other hand through the tracks to be soldered.
It can be seen however that this first execution entails problems in different respects. First of all, the heating of the layer of flexible, plastic material in contact with the hot bar can lead to its deterioration. This is especially marked as, in order to obtain sufficient heating of the solder paste, the energy that must be furnished to the layer of flexible plastic material is greater than what it would be in the event of direct contact with the tracks to be soldered. These problems are aggravated if the layer of flexible plastic material in contact with the hot bar is be covered with a second layer of conductive track, for example made of copper, this metal being a good heat conductor.
Besides, according to this first technique, excess solder paste cannot be removed by going back on either side of the different soldering tracks: this paste is indeed positioned directly on the plane of the tracks to be soldered and at the level of the printed circuit to which the FPC Board is soldered by the presence of the layer of flexible plastic material in contact with the hot bar. Thus, if there is excess solder paste, a short circuit can appear between two adjacent tracks. Such a phenomenon then leads to additional constraints in the production of the complete electronic system, and thus has an impact on its final cost.
For these different reasons, it is often preferable to move towards a different type of execution of hot-bar soldering in which the two layers of flexible plastic material are both perforated as is the second layer of conductive tracks, if necessary, at the track portion to be soldered (hot-bar soldering of the open-windowed flex type or exposed-lead flex type). Thus, the hot bar is applied directly to the portion of tracks to be soldered which, in this case, are entirely bared, thereby improving the heat transfer. The problems related to the removal of excess solder paste are also resolved (since the holes between the tracks serve to remove excess solder paste).
One drawback of this second type of implementation is that the tracks of the FPC board that were soldered are now accessible. Indeed, since the two layers of flexible plastic material, as well as possibly the second layer of conductive tracks, have been perforated to enable the hot bar to be applied directly to the tracks of the FPC Board, the electrical signals transiting between the FPC Board and the electronic circuit to which it has just been soldered are accessible and can be spied upon. This situation is a particular source of problems in certain fields of application where such access is not authorized, for example in devices that manage confidential data, such as payment terminals, payment card readers etc. Indeed, sensitive signals transit between the printed circuits boards. These sensitive signals for example represent a bank account, a secret code such as for example a PIN (personal Identification number). It is therefore necessary to prevent access thereto.
Besides, known FPC Boards used in such fields of application classically have means for detecting attempted intrusion and access to the signals that transit through the tracks that they include (e.g. security meshes comprising conductive tracks disposed on a second copper layer of the FPC board as described here above). It is thus worthwhile to obtain the same function with regard to the signals transiting through the solder points, i.e. the capacity to be able to detect an attempt to access these signals, and to do so while maintaining the cost of the final solution.
There is thus a need for an FPC Board that can be soldered by a hot-bar soldering method of the “window” or “exposed lead” type while at the same time preventing access to the electrical signals transiting between the FPC board and the electronic circuit to which it has just been soldered.
There is also need for such an FPC Board that provides means for detecting an attempt to access the electrical signals thus protected after soldering.
Finally, there is a need for such an FPC board that leads to a comprehensive solution for controlled costs and compactness.